Segment Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
Segment Reporting Information [Line Items]
Earned premiums	$ 1,071	$ 992	$ 3,157	$ 2,877
Fee revenues	3	2	9	6
Investment income, net of expenses	138	133	409	392
Realized investment gains, net	65	22	101	77
Other	3	3	7	7
Revenues	1,280	1,152	3,683	3,359
Income (loss) before income taxes	259	182	485	547
Identifiable assets	18,474		18,474		17,662
Consolidated Property and Casualty Insurance Entity
Segment Reporting Information [Line Items]
Identifiable assets	2,434		2,434		2,455
Life Insurance Segment
Segment Reporting Information [Line Items]
Identifiable assets	1,269		1,269		1,225
Investments Segment
Segment Reporting Information [Line Items]
Identifiable assets	14,322		14,322		13,618
Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	720	680	2,126	1,956
Fee revenues	1	1	3	2
Revenues	721	681	2,129	1,958
Income (loss) before income taxes	67	43	97	135
Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Earned premiums	263	244	775	712
Fee revenues	1	0	2	1
Revenues	264	244	777	713
Income (loss) before income taxes	14	14	(33)	33
Operating Segments | Excess and Surplus Lines Insurance
Segment Reporting Information [Line Items]
Earned premiums	42	30	109	85
Income (loss) before income taxes	13	4	22	5
Operating Segments | Life Insurance Segment
Segment Reporting Information [Line Items]
Earned premiums	46	38	147	124
Fee revenues	1	1	4	3
Revenues	47	39	151	127
Income (loss) before income taxes	(4)	0	(5)	10
Operating Segments | Investments Segment
Segment Reporting Information [Line Items]
Investment income, net of expenses	138	133	409	392
Realized investment gains, net	65	22	101	77
Revenues	203	155	510	469
Income (loss) before income taxes	182	134	448	409
Corporate and Other Non-Segment
Segment Reporting Information [Line Items]
Other	3	3	7	7
Income (loss) before income taxes	(13)	(13)	(44)	(45)
Identifiable assets	449		449		364
Commercial Casualty | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	237	221	695	636
Commercial Property | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	186	161	537	460
Commercial Auto | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	133	124	391	355
Workers' Compensation | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	93	95	280	270
Specialty Packages | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	27	38	93	114
Management Liability and Surety | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	32	30	94	89
Machinery and Equipment | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	12	11	36	32
Personal Auto | Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Earned premiums	121	112	354	328
Home Owner Segment | Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Earned premiums	110	103	330	298
Other Personal Lines | Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Earned premiums	$ 32	$ 29	$ 91	$ 86